Why won't my doctor correct my vision to 6/5?

So I went for my routine contact lens aftercare eye checkup today, and the doctor started testing my eyes with different lenses to reevaluate my lens power.

At one point I could read the very last line of his test chart, bright and clear. But he changed lenses until I could only read up to the line above the last line, and wrote those (weaker) lenses in my prescription.

On asking he said my vision becomes 20/15 if he's using -5.75 lenses, and is 20/20 if he's using -5.5 lenses. 20/20 is the standard vision of a normal eye and he wouldn't prescribe me 20/15 lenses as this is "not required".

But what's the harm? Why wouldn't he correct my eyes to 20/15?

Now neither an optician nor an online contact lens seller is agreeing to give me -5.5 contact lenses, they will just give -5.25 as the prescription reads "Spectacle power:-5.50DSPH/No Cyl/Add 0, CL params: -5.25DSPH/No Cyl/BC:8.6/DIA:14.2/Alcon O2 Optix/Modality:planned monthly, VA:20/20" for both eyes.

Why won't he correct my eyes to 20/15? Why?

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